His son has been kidnapped and his daughter has been left injured on Halloween of 1981 and no amount of searching or spell casting can find him.(It's up to you as to why) He raises his daughter and when the time comes, she goes to Hogwarts but under a different name to avoid any kind of bullying she might get.
You can call her whatever, put her in any of the houses so as to fit your story. She can get along with Harry or not, it doesn't really matter. But they do have to get along in the end, it doesn't have to be that they're really close but they have to be civil, at least.
- Be set during one or more of the first six years (extra kudos for using Goblet of Fire and having them both in the tournament)
- Have a reveal about Snape having a daughter
- Have a reveal about Harry being Snape's kidnapped son
- Evil Dumbledore, or, at least, evil Dumbledore but with good intentions.
- Abusive Dursleys
- Have Lily as their biological mother, but doesn't have to.
- Have some sort of addiction to something (drugs or something else) for either Harry or the daughter, or both.
Would be interesting if you included things such as:
- A summer with all three Snapes living together
- A massive Wizarding Trial, if you have an evil Dumbledore
- Draco in a relationship with someone
- How Snape would deal with one of his children being severely injured
- Harry Potter died that night, and that is the reason Severus's son was taken